1. Field of the Invention
This invention relates generally to AC motor drives or controllers, and relates more particularly to a motor controller that provides four quadrant control of the torque, speed, or position of a motor at a controlled power factor.
2. Description of the Relevant Art
A prior controller for an AC induction motor uses switched inverters to control the currents applied to the stator windings of the motor. For a three-phase motor, the inverter includes six active switches arranged in a bridge configuration between a DC bus and the motor. A diode bridge is coupled between the DC bus and a utility source of AC power and charges the DC bus by rectifying the supplied AC electrical power. Field oriented control can be used to control the current supplied to the motor.
This type of controller has certain drawbacks. The diode bridge generates harmonic currents that distort the utility voltage. These harmonic currents require additional VA power from the utility source, thus reducing overall system efficiency.
Another drawback is that the diode bridge is not capable of returning power to the utility source. When the motor is running as a generator in the second and fourth quadrants of torque/speed plane, the power generated is shunted to ground through a DC bus resistor. Again, this results in low system efficiency.
Still another drawback is that the dynamic performance of the field oriented current controller at high speeds is degraded since the diode bridge limits the maximum DC bus voltage. This results in performance degradation at high speeds.